In recent years, as portable electronic devices such as mobile telephones and notebook personal computers become widespread, there is a large need for development of compact and lightweight non-aqueous electrolyte secondary batteries that have high energy density. Moreover, there is also a strong need for development of a high-output secondary battery as a motor drive battery, and particularly, as a battery for the power source of transport equipment.
As a secondary battery that satisfies this kind of need is a lithium-ion secondary battery. A lithium-ion secondary battery comprises an anode, a cathode and an electrolyte, and a material in which lithium can be desorbed and inserted is used as the active material for the anode and cathode.
Currently, much research and development is being performed related to lithium-ion secondary batteries, and of that, research of lithium-ion batteries that use lithium metal composite oxide having layered structure or spinel structure as the cathode material has been advancing as high-energy density batteries that are capable of 4V class high voltage.
Currently, as the cathode material of that kind of lithium-ion secondary battery, lithium metal composite oxides such as lithium cobalt composite oxide (LiCoO2) having a relatively simple composition, lithium nickel composite oxide (LiNiO2), which uses nickel that is less expensive than cobalt, lithium nickel manganese cobalt composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium manganese composite oxide (LiMn2O4) that uses manganese, and lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2) have been proposed.
Even of these cathode active materials, lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2), which is high capacity, has excellent thermal stability and does not use cobalt of which there are few reserves, has gained much attention in recent years. Lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2) is layered in the same way as lithium cobalt composite oxides and lithium nickel composite oxides, and nickel and manganese are included in transitional metal sites at basically a compositional ratio of 1:1 (see Chemistry Letters, Vol. 30 (2001), No. 8, p. 744).
Incidentally, as a condition for a lithium ion secondary battery to obtain good performance characteristics (high cyclability, low resistance, high output) cathode material comprising particles having a uniform and suitable particle size is required.
This is because, when a cathode material having a large particle size and low specific surface area is used, the reactive area with the electrolyte cannot be sufficiently maintained, so the reaction resistance rises, and it is not possible to obtain a battery having high output. Moreover, when a cathode material having a wide particle size distribution is used, the voltage applied to the particles in the electrode become uneven, and when the battery is repeatedly recharged, small particles selectively deteriorate, and the capacity decreases.
In aiming for high output of a lithium-ion secondary battery, shortening the distance between the cathode and anode is effective, so preferably the cathode plate is made to be thin, and from this aspect as well, using cathode material having a small particle size is useful.
Therefore, in order to improve the performance of the cathode material, it is important that lithium nickel manganese composite oxide, which is a cathode active material, be manufactured so that the particle size is small and uniform.
Lithium nickel manganese composite oxide is normally manufactured from composite hydroxide, so in order to make the lithium nickel manganese composite oxide particles small with a uniform particle size, it is necessary to use a composite hydroxide as the raw material that has small particles with a uniform particle size.
In other words, in order to improve the performance of the cathode material and manufacture a high-performance lithium-ion secondary battery as a final product, it is necessary to use a composite hydroxide that comprises particles having a small particle size and narrow particle distribution as the composite hydroxide that will become the raw material of the lithium nickel manganese composite oxide used in forming the cathode material.
As a nickel manganese composite hydroxide that is used as the raw material of a lithium nickel manganese composite oxide, manganese nickel composite hydroxide particles are proposed in JP2004-210560(A) which are composite hydroxide particles having a manganese to nickel ratio of 1:1, with an average particle size of 5 to 15 μm, tap density of 0.6 to 1.4 g/ml, bulk density of 0.4 to 1.0 g/ml, specific surface area of 20 to 55 m2/g, amount of sulfate contained being 25 to 45 weight %, and in X-ray diffraction, a ratio (I0/I1) of the maximum strength (I0) of the peak in the range 15≤2θ≤25 and the maximum strength (I1) of the peak in the range 30≤2θ≤40 of 1 to 6. The secondary particle surface and internal structure is formed in a netlike structure with fold-like walls of primary particles, with the space surrounded by the fold-like walls being relatively large.
Furthermore, as the manufacturing method, a method is disclosed in which, while keeping the amount of oxidation of manganese ions within a set range, a mixed aqueous solution of manganese salt and nickel salt having an atomic ratio of manganese and nickel of 1:1 is mixed and reacted with an alkaline solution in an aqueous solution having a pH or 9 to 13 with the existence of a complexing agent to cause coprecipitation of particles.
However, in the case of the lithium manganese nickel composite oxide and manufacturing method disclosed in JP2004-210560(A), although the structure of the particles is considered, it can be clearly seen in the disclosed electron micrograph that coarse particles and fine particles are mixed together in the obtained particles, and making the particle size uniform has not been considered.
On the other hand, in regards to the particle size distribution of lithium composite oxide particles, a lithium composite oxide has been disclosed in JP2008-147068(A) such that in the particle size distribution curve, the particles have a particle size distribution with an average particle size D50, which means the particle size of a cumulative frequency of 50%, of 3 to 15 μm, a minimum particle size of 0.5 μm or greater, and a maximum particle size of 50 μm or less, and where in the relationship between average particle size D10 at a cumulative frequency of 1.0% and D90 at a cumulative frequency of 90%, the ratio D10/D50 is 0.6 to 0.9, and the ratio D10/D90 is 0.30 to 0.70. It has also been disclosed that this lithium composite oxide has high repletion, excellent charge and discharge characteristic and high output characteristic, and does not easily deteriorate even under conditions of a large charge and discharge load, so by using this lithium composite oxide, a non-aqueous electrolyte lithium ion secondary battery having excellent output characteristics and little deterioration of cyclability can be obtained.
However, the lithium composite oxide disclosed in JP2008-147068(A) includes fine particles and coarse particles as seen from the fact that it has a minimum particle size 0.5 μm or greater and a maximum particle size of 50 μm or less with respect to an average particle size of 3 to 15 μm. The above particle size distribution that is regulated by D10/D50 and D10/D90 is not a narrow particle size distribution range. In other words, the lithium composite oxide of JP2008-147068(A) can be said to have sufficiently high uniformity of particle size, and when that lithium composite oxide is used, an improvement in performance of the cathode material cannot be expected, and it is difficult to obtain a non-aqueous electrolyte lithium-ion secondary battery having sufficient performance.
Moreover, a method for manufacturing a composite hydroxide that will become the raw material for a composite oxide aimed at improving the particle size distribution has been disclosed. In JP2003-086182(A), in a method for manufacturing a cathode active material for a non-aqueous electrolyte battery, a method for obtaining a hydroxide or oxide as a precursor is disclosed in which an aqueous solution containing two or more kinds of transition metal salts or two or more kinds of aqueous solutions of different transition metal salts is put into a reaction vessel together with an alkaline solution, and co-precipitation is performed while causing the solution to coexisting with a reducing agent or by passing an inert gas though the solution.
However, the technology disclosed in JP2003-086182(A) is for recovery while classifying the generated crystals, so in order to obtain a material having uniform particle size, strictly managing the manufacturing conditions is considered to be necessary, so production on an industrial scale is difficult. Moreover, even though it is possible to obtain crystal grain having a large grain size, obtaining small particles is difficult.
Furthermore, in order to make a battery with high output, increasing the size of the reactive area without changing the particle size is effective. In other words, by making particles that are porous, or that have a hollow particle structure, it is possible to increase the surface area that contributes to the battery reaction, and it is possible to reduce the reaction resistance.
For example, in JP2004-253174(A) cathode active material for a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide at least having a layered structure is disclosed wherein the lithium transition metal composite oxide comprises hollow particles having a shell section on the outside and a hollow section on the inside of the outer shell section. Also disclosed is that this cathode active material for a non-aqueous electrolyte secondary battery has excellent battery characteristics such as cycle characteristics, output characteristics, thermal stability and the like, and can suitably be used for a lithium-ion secondary battery.
However, JP2004-253174(A) makes no mention of particle size of the cathode active material, while the cathode active material comprises hollow particles and therefore these particles are expected to have a greater specific surface area than solid particles. Therefore, improvement in reactivity with the electrolyte due to an increase in specific surface area can be expected, however, the effect on the migration distance of the lithium ions due to making the particles smaller is not clear; and a sufficient improvement in output characteristics cannot be expected. Furthermore, in regards to the particle size distribution, the particle size distribution is considered to be the same as in conventional cathode active material, so selective deterioration of minute particles due to uneven voltage that is applied inside the electrodes occurs, and there is a strong possibility that there will be a drop in battery capacity.
As described above, currently neither a lithium composite oxide that can sufficiently improve the performance of a lithium-ion secondary battery, nor a composite hydroxide that will become the raw material for that composite oxide have been developed. Moreover, after investigating various methods for manufacturing composite hydroxides, currently a method that is capable on an industrial scale to manufacture a composite hydroxide that can become the raw material for a composite oxide capable of improving the performance of a lithium-ion secondary battery has not been developed. In other words, a cathode active material having particles with a small and uniform particle size, and that have a large reactive area, for example having a hollow structure, remains to be developed, and a method capable of industrially manufacturing that kind of cathode active material is desired.